Are Humans Descended from a Knuckle‐Walking Ancestor?

Abstract

A central topic in human evolution pertains to the emergence of habitual bipedalism [i.e., upright walking (UW)], a behavioural characteristic that defines the human lineage and is unique among primates. To pinpoint the factors that led to the appearance of UW, one has to reconstruct the locomotor behaviour of the last common ancestor of humans and their closest living relatives, the chimpanzees. Chimpanzees and gorillas adopt a peculiar mode of locomotion when travelling on the ground, knuckle‐walking (KW). This article evaluates the evidence for and against the hypothesis that humans have descended from a KW ancestor. The conflicting nature of this evidence does not allow firm conclusions, but future avenues of research are proposed.

Key Concepts

  • The context for the emergence of habitual bipedalism/upright walking is a key topic in human evolutionary studies.
  • Bipedalism/upright walking is a characteristic that defines the human lineage.
  • Some evidence suggests that the ancestor of the earliest hominins was a knuckle‐walker.
  • However, other evidence appears to contradict this possibility.
  • The role of knuckle‐walking in human evolution will be better understood as more data about the functional anatomy of knuckle‐walking is gathered and as more fossils are described that shed light on the morphology of the last common ancestor of humans and chimpanzees.

Keywords: knuckle‐walking; bipedalism; upright walking; human–chimpanzee last common ancestor; Ardipithecus

Figure 1. Radiographs in medio‐lateral view of the wrist of a common chimpanzee. (a) Extended wrist (support phase). (b) Flexed wrist (swing phase). Reproduced from Richmond and Strait (2000) © Nature Publishing Group.
Figure 2. Evolutionary scenarios for the evolution of bipedalism. (a) Arboreal climbing/clambering ancestor scenario. (b) Knuckle‐walking ancestor scenario. Blue: arboreal climbing; green: knuckle‐walking; red: bipedalism. The revival of the Knuckle‐Walking Hypothesis was spurred by the analysis of molecular data that revealed the existence of a human–chimp clade (to the exclusion of gorillas) as depicted in the phylogenetic trees in (a) and (b).
Figure 3. Illustration of the concept of morphological integration. Each shape and colour represents a character that evolves (arrow) to a new state represented by a new shape and/or colour. When counted separately, it appears that all six traits are independently experiencing evolutionary change. However, if the characters are integrated such that all of the traits evolve in response to a single evolutionary phenomenon, then only a single evolutionary change is needed to explain the evolution of the complex of traits.
Figure 4. Hand and wrist postures during knuckle walking in chimpanzees and gorillas. Reproduced with permission from Kivell and Schmitt (2009) © PNAS.
Figure 5. Palmar view (i.e. from the front) of the distal end of the radius (a forearm bone) that articulates with the bones of the wrist. Arrows indicate the presence of radial dorsal ridge. Reproduced from Richmond and Strait (2000) © Nature Publishing Group.
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Further Reading

Harrison T (2010) Apes among the tangled branches of human origins. Science 237: 532–534.

Wang W, Abboud RJ, Günther MM and Crompton RH (2014) Analysis of joint force and torque for the human and non‐human ape foot during bipedal walking with implications for the evolution of the foot. Journal of Anatomy 225 (2): 152–166.

Wood, BA (2001) Human Evolution: Early Radiations. eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net. 10.1038/npg.els.0001648

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How to Cite close
Makedonska, Jana A, and Strait, David S(May 2015) Are Humans Descended from a Knuckle‐Walking Ancestor?. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026161]